Method of Determining Surface Pore Mouth Diameter Distribution of Porous Material

ABSTRACT

A method of determining surface pore diameter distribution of a porous material is provided. A compressed gas is charged from backside of a sample of a porous body, a surface of the sample is immersed into a liquid wetting agent, and a gas pressure is regulated from high to low, such that pore mouths of the surface of the porous body are blocked by the wetting agent in an order from small to large pore sizes, and a gas flow rate is more rapidly decreased. When the pressure is decreased to a value, the gas flow rate is reduced to zero and the pressure at this time is corresponding to the largest pore mouth size of the surface of the porous body. The pore size distribution is determined by comparing the obtained gas pressure—flow rate curve (wet curve) with a gas pressure—flow rate curve (dry curve) obtained for the sample under a dry state. The pore size and the pore distribution are calculated based on the conventional bubble pressure method. The measurement of the surface pore distribution of the porous body is of great significance for investigating the support performance of the porous body.

FIELD OF THE INVENTION

The present invention relates to a method of determining surface pore diameter distribution of a porous material, which is used to measure a diameter of a pore mouth instead of a pore throat (narrowest point) of a pore passage.

RELATED ART

Porous materials have a number of excellent properties due to many internal pores, can be categorized into those with a uniform pore structure and those with a gradient pore structure, and have been widely applied in many processes of petrochemistry, food, construction, metallurgy, and aerospace industries, for example, for separation and purification, gas distribution, catalysis, silencing and damping, shielding, heat exchange, electrochemistry, and the like. In addition, in recent years, more attention has been paid to composite membranes formed by applying other functional materials on the surface of a porous base material. When a porous material is used in a filtration process, the most concerned issue is the pore size. The “pore size” here mainly refers to the narrowest point in a pore passage, i.e. “pore throat”, as shown in FIG. 1. However, when a coating is prepared on the surface of a porous material, because the coating is located at the pore mouth, the size of the pore throat is much smaller than that of the pore mouth. Undoubtedly, at this time, the pore distribution of the pore mouth (not pore throat) has a direct influence on the quality of the coating. A typical example is the preparation of Pd composite membranes.

Pd membranes (including Pd alloy membranes) have good hydrogen permeability, and have been used in production of super-pure hydrogen for several decades. Except for hydrogen and isotopes thereof, any other gas cannot penetrate through the Pd membranes. The hydrogen permeability of the Pd membranes is inversely proportional to its thickness, and the Pd membranes cannot be too thin, in order to maintain a sufficient mechanical strength. The commercial Pd membranes mostly have a thickness of 100 μm or greater, and further decrease in membrane thickness results in too low mechanical strength of the Pd membranes. An ideal solution is that a Pd membrane is loaded onto a surface of a porous base (for example, a porous ceramic and a porous stainless steel) to form a composite Pd membrane, whereby the membrane thickness can be reduced to only several microns and thus the hydrogen permeability of the Pd membrane can be improved by one order of magnitude. Moreover, the decrease of the membrane thickness also reduces the consumption of the noble metal. The disadvantages of the composite Pd membrane are in that membrane defects may easily occur, and the larger the surface pore of the base is, the more the need for increasing the membrane thickness to control the membrane defects is [Mardilovich I P, Engwall E, Ma Y H. Dependence of hydrogen flux on the pore size and plating surface topology of asymmetric Pd-porous stainless steel membranes. J. Membr. Sci., 2002, 144: 85-89]. In order to prepare a Pd membrane completely without hole defects, a particular attention needs be paid to the maximal pore size, and not the mean pore size of the porous body, more specifically, to the maximal size of the pore mouth [Yu Jian, Hu Xiaojuan, Huang Yan. Modifications with ceramics on a surface of a porous stainless steel and loaded hydrogen-permeable Pd membranes, Progress in Chemistry, 2008, 20(7/8): 1208-1215].

There are many methods for determining the pore size distribution of a porous material [Hernandez A, Calvo J I, Prádanos P, Tejerina F. Pore size distributions of track-etched membranes; comparison of surface and bulk porosities Colloids and Surfaces A. 1998, 138: 391-401.] [Zhang Qing, Zhang Zhengde, Wei Hairong. Characterization of filtration accuracy for porous materials. Filtration and Separation, 2000, 10(1): 33-37.], including the mercury porosimetry method, the bubble pressure method, the liquid-liquid exclusion method, the suspension filtration method, the gas permeation method, the direct observation from cross-section, and the like.

The mercury porosimetry method involves pressing mercury into a dried porous sample by an external force, determining the change of the volume of mercury entered into the sample with the external pressure, and accordingly determining the pore size distribution of the sample. This method, however, also includes semi-permeable pores having no filtration function because it is used to detect the pore size distribution of the entire porous body, thus, the determination results have less value as reference for the actual filtration effect. Moreover, the mercury porosimetry method is not suitable for filtration materials with a gradient pore size structure at all.

The principle of the bubble pressure method (or termed the bubble point pressure method) [ASTM Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test F316-2003.] [ISO Permeable sintered metal materials determination of bubble test pore size 4003-1990.] [GB/T 1967-1996 Test method for pore diameter of porous ceramics] [GB 5249-1985 Permeable sintered metal materials—Determination of bubble test pore size] [Huang Pei, Xing WeiHoing, Xu Nanping et. al. Study on determination of the pore size distribution of an inorganic microfiltration membrane by the gas bubble pressure method, Technology of Water Treatment, 1996, 22(2): 80-84.] is in that, when the pore passage is blocked by the wetting agent, due to the action of the surface tension of the wetting agent, application of a certain pressure is required for pore opening, and further, the smaller the pore size is, the larger the required pressure is. Therefore, the gas pressure can be gradually increased to open the pore passages in an order from large to small diameters. The first pore being opened is the largest pore, i.e. “bubble point”. This method requires determining the relationship between the pressure and the gas flow rate for the porous material under a dry and wet state, and calculating the pore size distribution based on a certain model. Notably, all of the pore sizes measured with the bubble pressure method refer to the diameter of the narrowest point, i.e. “pore throat” of the pore passage. In most cases, the pores are irregular in shape, and the so-called pore size actually refers to the diameter of a circle with the same area as the cross-sectional area at the pore throat.

The principle of the liquid-liquid exclusion method is the same as that of the bubble pressure method, except that another liquid immiscible with the wetting agent is substituted for the gas as the pore-opening agent. Also the pore-throat size distribution is measured with this method.

The suspension filtration method involves using a suspension of spherical particles with a certain particle size as a medium, passing the suspension through a porous material under a laminar flow condition, and measuring the largest particle diameter contained therein as the maximal pore size of the porous material. Unlike the definition of the pore size in the bubble pressure method, the pore size here refers to the diameter of an inscribed circle of the pore throat, and for a non-circular pore passage, the pore size measured with the bubble pressure method is larger than that measured with this method.

Except for the mercury porosimetry method, the above methods cannot provide the pore distribution information on the pore mouths, although they can measure the pore distribution of the pore throats of the porous materials and are of great value as reference for studying the filtration performance of these porous materials. At present, there has been no idea for studying the surface pore mouth diameter. Direct observation with a microscope, for example, SEM, can be used in laboratory researches; however, this method has a small visual field, generally can only be used for observation of a tiny sample, and also is incapable of effectively detecting the largest pores. Therefore, utility and value as reference of the method are very limited.

SUMMARY OF THE INVENTION

The present invention is directed to a method of determining surface pore mouth diameter distribution of a porous material.

The technical solution of the present invention is as follows:

A compressed gas is charged from backside of a sample of a porous body, a surface of the sample is immersed into a wetting agent, and a gas pressure is regulated from high to low. The smallest pore mouth is closed first due to the surface tension of the wetting agent. With decrease of the gas pressure, more pore mouths are closed by the wetting agent in an order from small to large pore sizes, and the last pore to be closed has the largest pore size of the pore mouth. Finally, the pore size distribution can be calculated by comparing the relationship of the pressure and the gas flow rate under a dry state and under a wet state, based on a certain model. Compared to the conventional bubble point method, both of them utilize the relationship between surface tension and pore size of the capillary bore for measuring the pore size, that is, the conventional bubble point method utilizes the corresponding relationship of pore-opening pressure and pore size, while this method utilizes the corresponding relationship of pore-closing pressure and pore size; however, the data processing of the two methods is the same. The difference lies in that the conventional bubble point method is to measure the pore throat size, while this method is to measure the pore mouth size. In operation, the former requires previously soaking the sample in the wetting agent and then gradually increasing the gas pressure to measure the pore-opening effect, while the latter requires passing the compressed gas over the sample at first and then immersing the surface to be tested into the wetting agent, and gradually decreasing the gas pressure to measure the pore-closing effect.

When a capillary bore of a certain size contacts with the wetting agent and the pore mouth is blocked by the wetting agent so that pore-closing occurs, due to the action of surface tension, the liquid level of the pore mouth will generate the following pressure intensity on the interior of the pore:

$\begin{matrix} {p = \frac{4\; \sigma \; \cos \; \theta}{d}} & \lbrack 1\rbrack \end{matrix}$

where σ indicates the surface tension of the wetting agent, θ indicates a contact angle [Liu Peisheng, Ma Xiaoming Ed. A Detection Method for Porous Materials. Beijing: Metallurgical Industry Press, 2006: 60-61], and d indicates the diameter of the pore mouth. When there is very good wettability between the porous body and the wetting agent, θ≈0. When the pore mouth has not been blocked, the gas pressure P>p; when the pore mouth is to be blocked, P=p, that is:

$\begin{matrix} {d \approx \frac{4\; \sigma}{P}} & \lbrack 2\rbrack \end{matrix}$

In the determination, although because the porous body is placed below the liquid level, the static pressure of the wetting agent can partially counteract the gas pressure, in the actual operation, the sample is generally located below the liquid level at a shallow position, and thus this part of the static pressure is negligible. Otherwise, the formula above should be changed to:

$\begin{matrix} {d = \frac{4\; \sigma}{P - {\rho \; {gh}}}} & \lbrack 3\rbrack \end{matrix}$

where ρ indicates the density of the wetting agent, g indicates a constant of 9.8 N/kg, and h indicates the depth by which the porous body is immersed into the wetting agent.

A particular technical solution of the present invention is a method of determining the surface pore mouth diameter distribution of a porous material, comprising: charging a compressed gas from backside of a sample of a porous body; immersing a surface of the sample into a liquid wetting agent; regulating a gas pressure from high to low and measuring a gas flow rate, in which when the pressure is decreased to a value, the gas flow rate is reduced to zero and the pressure at this time is corresponding to the largest pore mouth size of the surface of the porous body, or controlling the gas flow rate from high to low to regulate the pressure, in which when the gas flow rate is regulated to zero, the pressure is corresponding to the largest pore mouth size of the surface of the porous body; and calculating the pore size distribution by comparing the measured pressure—flow rate curve (wet curve) with a gas pressure—flow rate curve (dry curve) measured for the sample under a dry state, in which half of the flow rate in the dry curve is plotted against the gas pressure to give a half-dry curve, and the pore size corresponding to the pressure at which the half-dry curve intersects with the wet curve is a mean pore size of the pore mouth (also termed mid-flux pore, that is, passages with a pore mouth larger than this pore size and passages with a pore mouth smaller than this pore size each have a contribution of 50% for the gas flux).

The dry curve can be measured from a low pressure to a high pressure or from a high pressure to a low pressure. In the measurement of the dry or wet curve, the flow rate can be regulated by controlling the gas pressure, or the pressure can be regulated by controlling the gas flow rate. Moreover, it has been found in practice that, the regulation of the pressure by controlling the gas flow rate is more convenient in operation.

The porous material to be tested is a ceramic, glass, metal, or plastic material, or a composite material with a surface to be tested made of these materials. The compressed gas used in the measurement is preferably air, nitrogen, or argon. The wetting agent is non-toxic, less viscous, and chemically inert, and has good wettability for the porous material. In the measurement of the wet curve, the pressure is regulated from high to low, until the gas flow rate is zero. The initial pressure is determined depending on the actual situation, including the surface pore size of the sample, the selected wetting agent, the measuring ranges of the pressure and flow rate devices, the pressure of the gas source, and so on. It can be seen from the formula [1] that, the pore size is inversely proportional to the pressure, and the smaller the pore size is, the higher the pore-opening pressure is. For example, when anhydrous ethanol (σ=22.3×10⁻³ N/m) is used as the wetting agent, only 0.18 MPa of the gas pressure is required in order to turn a pore of 0.5 μm into an open state, and 1.78 MPa is required for a pore of 0.05 μm. Although the higher the initial pressure is, the wider the range of the detectable pore sizes is, in fact, a too high pressure results in time consumption and a waste of gas. In the actual operation, if the contribution of a part of small pores on the flux is negligible, the initial pressure can be reduced to a great extent. Of course, the use of a wetting agent with a smaller surface tension also is an effective method for reducing the starting pressure. For example, for a hydrophilic material with a pore mouth diameter of 5-200 μm, water is preferably used as the wetting agent. For a smaller pore mouth diameter, a wetting agent with a smaller surface tension, such as alcohols and ketones, is appropriately used. For the wet curve, the initial pressure is preferably 0.2-0.5 MPa, and it is preferred to use some commercially available specific wetting agents, such as Galden HT230, Porewick, Galwick, Silwick, and the like. For an unknown sample, an initial pressure can be set at first, and if it is found during the measurement that the pressure is not enough, after the sample is dried, a higher initial pressure can be set or other wetting agents are used instead for re-measurement.

For the conventional bubble point method or for the method of the present invention, the original data obtained from experiments are two pressure—flow rate curves measured for the sample under a dry/wet state, i.e. a wet curve and a dry curve. All of the results (maximum pore size, mean pore size, pore size distribution, most probable mean pore size, and the like) are calculated based on the two curves. The mathematical model and the calculation method both are not within the scope of the present invention, and the details can be seen in the literature [Huang Pei, Xing Weihong, Xu Nanping et. al. Study on Determination of Pore Size Distribution of Inorganic Microfiltration Membrane by Gas Bubble Pressure Method. Technology of Water Treatment, 1996, 22(2): 80-84]. In consideration of the two most concerned parameters, namely, maximum pore size and mean pore size, only these two parameters will be calculated in embodiments of the present invention.

The beneficial effects of the present invention are:

1. The diameter of the pore throat is measured with the conventional bubble pressure method, liquid-liquid exclusion method, or suspension filtration method; the present invention provides a method of determining surface pore mouth diameter distribution of a porous material, which is of great significance for investigating the support performance of the porous material.

2. The direct observation of the surface pore mouth through the microscopy technique has the disadvantages of a small visual field and being incapable of effectively detecting the maximum pore, and thus utility and value as reference are very limited; this method has better utility and causes no damage to the sample.

3. The device used in the present invention is similar to that in the conventional bubble pressure method, is easy to construct, simple to operate, and suitable for the detection of products in scientific research and production processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the pore throat and pore mouth of the pore passage in a porous material (a. pore mouth, and b. pore throat);

FIG. 2 shows a testing device for a plate porous material, in which: 1. compressed gas cylinder, 2. mass flowmeter, 3. pressure gauge, 4. plate sample cell, 5. sheet sample, 6. wetting agent, 7. water tank;

FIG. 3 shows a testing device for a tube porous material, in which: 1. compressed gas cylinder, 2. mass flowmeter, 6. wetting agent, 7. water tank, 8. pressure controller, 9. tube sample cell, 10. tube sample;

FIG. 4 shows a gas flow rate—pressure curve of a porous ceramic sheet;

FIG. 5 shows a gas flow rate—pressure curve of the microporous side of a stainless steel membrane with a gradient structure; and

FIG. 6 shows a gas flow rate—pressure curve of the macroporous side of a stainless steel membrane with a gradient structure, in which:

dry curve,

wet curve,

half-dry curve.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

i. A sample to be tested was a porous ceramic sheet having a diameter of 30 mm and a thickness of 2 mm. The sample was washed with 1 ml/L of a diluted HCl solution followed by DI water for about 5 min, and dried.

ii. The testing device, as shown in FIG. 2, was composed of a compressed gas cylinder (1), a mass flowmeter (2), a pressure gauge (3), a sample cell (4), a sample (5), and a water tank (7). The sample was placed into the sample cell (4) with the surface to be tested facing outwards, and two sides were respectively sealed by an annular silicone rubber pad with an outer diameter of 30 mm, an inner diameter of 10 mm and a thickness of 2 mm. The mass flowmeter (2) was regulated to gradually increase the gas flow rate from zero, and the gas flow rate and the pressure were recorded. When the flow rate was 5 L/min, the pressure was 184 kPa. At this time, the flow rate was plotted against the pressure to give a dry curve, and half of the flow rate was plotted against the pressure to give a half-dry curve. The results are shown in FIG. 4.

iii. Into the water tank (7) as shown in FIG. 2, DI water (6) (σ=72.9×10⁻³ N/m) was added as a wetting agent. The gas flow rate was regulated to 5 L/min. The sample was immersed into water below 1-2 cm, and slightly shaken to ensure that the sample surface sufficiently contacts with water. After it was stable, the pressure gauge (3) shows 187.3 kPa; the gas flow rate was down-regulated to 4 L/min, and after it was stable, the pressure was 163.3 kPa. In this manner, the gas flow rate was continually down-regulated to decrease the gas pressure, and the values of the flowmeter and the pressure gauge were recorded. When bubbling occurred in only a few of pores on the sample surface, the gas was completely closed, and after the last bubble disappeared, the pressure of the system remained, i.e. bubble point pressure, was 11.2 kPa, and the corresponding pore size, i.e. the largest pore mouth, was 26 μm. The data was plotted to give a wet curve, as shown in FIG. 4. The pressure at which the wet curve intersects with the half-dry curve was 45.6 kPa, and the corresponding pore size was calculated to be 6.4 μm according to the equation

${d \approx \frac{4\sigma}{P}},$

which was the mean pore size of the surface pore mouth of the sample.

iv. With DI water as a wetting agent again, the CFP-1100A porometer by PMI Inc. (US) was used to measure the pore throat of the sample, and the results were as follows: the mean pore size was 3 μm and the maximum pore size was 6.3 μm. Both of them are smaller than the pore mouth diameter measured with this method, which conforms to the fact that the pore mouth is larger than the pore throat.

Embodiment 2

i. The sample was a SS-316L stainless steel membrane with a gradient pore structure having a diameter of 30 mm and a thickness of 2 mm, composed of a macroporous support and a pore-size controller with a thickness of 200 μm. The sample was washed with 1 ml/L of a diluted HCl solution, a 0.5 mol/L diluted NaOH solution, and then DI water for about 5 min, and dried.

ii. In this case, the membrane surface of the sample with a small pore size was to be measured. The determination for the data of the dry curve, half-dry curve, and wet curve were the same as those in the steps (ii), (iii) and (iv) of Embodiment 1, and the results are shown in FIG. 5. The mean pore size and the maximum pore size of the surface pore mouth of the membrane were measured to be 13.9 μm and 62.1 μm respectively.

iii. The mean pore size and the maximum pore size of the pore throat of the sample were measured in the same manner as the step (v) of Embodiment 1, and were 9.1 μm and 41.7 μm respectively.

Embodiment 3

i. The sample to be tested was the same as that in Embodiment 2, except that the support side of the sample was to be measured.

ii. Other operations were the same as those in Embodiment 2. The results are shown in FIG. 6. The mean pore size and the maximum pore size of the surface pore mouth of the support were measured to be 64.8 μm and 171.5 μm respectively.

iii. It can be seen from the results of Embodiments 2 and 3 that, the pore size of the support surface is much larger than that of the membrane surface for the same sample, which conforms to the actual situation of the sample, and indicates that this method is reasonable.

Embodiment 4

i. The sample to be tested was a porous stainless steel sheet having a diameter of 30 mm and a thickness of 1.5 mm. The pre-treatment was the same as that in the step (i) of Embodiment 2.

ii. Ethanol was selected as a wetting agent (σ=22.3×10⁻³ N/m), and other operations were the same as those in the steps (ii), (iii) and (iv) of Embodiment 1. The mean pore size and the maximum pore size of the surface pore mouth of the sample were measured to be 4.9 μm and 12.1 μm respectively.

iii. The step was the same as the step (v) of Embodiment 1, but the Porewick wetting agent by PMI Inc. (US) was used. The mean pore size and the maximum pore size of the pore throat of the sample were measured to be 0.82 μm and 5.16 μm respectively.

Embodiment 5

i. The sample was a porous stainless steel-tube filter element having a length of 10 mm, an outer diameter of 12 mm, and an inner diameter of 9 mm. The mean pore size and the maximum pore size of the pore throat of the sample were measured with the bubble pressure method to be 4.8 μm and 7.0 μm respectively.

ii. In this case, the outer-surface pore of the sample was to be measured. The testing device, as shown in FIG. 3, was composed of a compressed gas cylinder (1), a mass flowmeter (2), a pressure controller (8), a tube sample cell (9), a tube sample (10), and a water tank (7). One end of the sample was blocked, and DI water was used as a wetting agent. The pressure controller (3) was regulated to gradually increase the gas pressure from zero, and the gas pressure and the flow rate were recorded. When the pressure was 100 kPa, the flow rate was 2.2 L/min. At this time, the flow rate was plotted against the gas pressure to give a dry curve, and half of the flow rate was plotted against the gas pressure to give a half-dry curve.

iii. Into the water tank (7) as shown in FIG. 3, DI water (6) (σ=72.9×10⁻³ N/m) was added as a wetting agent. The pressure controller was regulated to obtain a gas pressure of 100kPa. The sample was immersed into water below 1-2 cm, and slightly shaken to ensure that the sample surface sufficiently contacts with water. After it was stable, the flowmeter (2) shows 2 L/min; the gas pressure was down-regulated to 80 kPa, and after it was stable, the flow rate was 1.44 L/min. In this manner, the gas pressure was continually down-regulated to decrease the gas flow rate, and the values of the flow rate and the pressure were recorded. When bubbling occurred in only a few of pores on the sample surface, the regulation rate of the pressure was slowed down, and when the pressure was 10.1 kPa, the last bubble disappeared and the flow rate was zero. At this time, the corresponding pore size, i.e. the largest pore mouth, was 28.9 μm. The data was plotted to give a wet curve. The pressure at which the wet curve intersects with the half-dry curve was 40 kPa, and the corresponding pore size was calculated to be 7.3 μm according to the equation

${d \approx \frac{4\sigma}{P}},$

which was the mean pore size of the surface pore mouth of the sample. 

1. A method of determining surface pore mouth diameter distribution of a porous material, comprising: charging a compressed gas from backside of a sample of a porous body; immersing a surface of the sample into a liquid wetting agent; adjusting a gas pressure from high to low and measuring a gas flow rate, wherein when the pressure is decreased to a value, the gas flow rate is reduced to zero and the pressure at this time is corresponding to a largest pore mouth size of the surface of the porous body, or controlling the gas flow rate from high to low to adjust the pressure, wherein when the gas flow rate is regulated to zero, the pressure is corresponding to the largest pore mouth size of the surface of the porous body; and calculating the pore size distribution by comparing a measured pressure—flow rate curve as a wet curve with a gas pressure—flow rate curve as a dry curve measured for the sample under a dry state, wherein half of the flow rate in the dry curve is plotted against the gas pressure to give a half-dry curve, and a pore size corresponding to the pressure at which the half-dry curve intersects with the wet curve is a mean pore size of the pore mouth.
 2. The method of determining surface pore mouth diameter distribution of a porous material according to claim 1, wherein the porous material is a ceramic, glass, metal, plastic material, or a composite material with a surface to be tested made of the material. 